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Glossary Ablation. Removal of material from an imploding pellet by melting and vaporization. Accretion. The process by which mass falls onto a condensed object. The infalling gas is heated to high temperatures as it accretes onto white dwarfs, neutron stars, and black holes. The x radiation from the heated gas provides a characteristic observational signature for these objects. Activation. Process of making a material radioactive by bombard- ment with neutrons or other nuclear particles. Active galactic nuclei. Active galaxies, which radiate much of their energy, intense continuum, and strong atomic lines in nonthermal forms such as thermal and synchrotron emissions, are observed to generate their power in very small regions called nuclei at their centers. Many astronomers believe that mass accretion onto massive black holes at the centers of active galaxies are responsible for the enormous energy outputs of Seyfert galaxies, radio galaxies, and quasars. It is possible that all galaxies have nuclei that are active to some degree. Adiabatic compression. Compression of plasma not accompanied by gain or loss of heat from the outside. In adiabatic compression, the plasma density and temperature should both increase. Air shower. The shower of energetic particles created when a single cosmic ray or gamma ray collides with the nucleus of an atom in the 283

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284 GLOSSAR Y upper atmosphere. By studying the number and types of particles created in an air shower, and the particle trajectories, it is possible to infer the energy and, with less accuracy, the direction of arrival of the very-high-energy cosmic-ray primary. Alive'n speed. The characteristic speed of low-frequency waves in conducting magnetohydrodynamic (MHD) fluids. The Alfven speed is proportional to the magnetic-field strength and inversely propor- tional to the square root of the mass density. For MHD fluids, the Alfven speed plays a role similar to the sound speed in ordinary fluids. Named for the Swedish Nobel Laureate, Hannes Alfven. Alfve'n waves. Waves of a much lower frequency than the ion cyclotron frequency, occurring in a plasma or in a conducting fluid immersed in a magnetic field and characterized by a transverse motion of the lines of force together with the plasma. Alpha particle. A positively charged particle made up of two neu- trons and two protons bound together; the nucleus of a helium atom. Alpha particles are produced by the deuterium-tritium fusion reac- tion and serve to sustain the plasma temperature in an ignited fusion reactor. Ambipolar diffusion. Diffusion process in which the electrons and ions escape to the walls at exactly the same rate. Ambipolar potential. An electrostatic potential generated by a plasma to maintain equal loss rates for electrons and ions. Anisotropy. The condition of having different properties in different directions, for example, in the velocity distribution of particles. Anomalous transport. Processes of energy and particle transport in plasmas that generally exceed classical rates and are associated with plasma instabilities and fluctuations. ANTARES. A CO2-laser facility in operation at the Los Alamo s National Laboratory. Antenna. A device for coupling radio-frequency power to electro- magnetic waves that may be launched into a plasma. Aspect ratio. The ratio of the major radius of a torus to the plasma, . . Or minor, radius. Aurora, auroras, aurora borealis, auroral zone. Also known as the northern (and southern) lights, the term aurora refers to light emissions that originate at 100-120 km altitudes above the surface of the Earth. The aurora are typically most frequent between 60 and 70 north and south geomagnetic latitudes. Auroral activity moves equatorward during magnetic storms. Aurora are created when electrons and protons, which can be accelerated to 10 keV or more

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GO OSSAR Y 285 in energy at altitudes of 5000-10,000 km, collide with upper- atmospheric neutral atoms. Auxiliary heating. Any form of plasma heating other than the intrinsic (ohmic) heating by the plasma currents themselves. Neutral-beam injection and radio-frequency heating are the most common types of auxiliary heating. Ballooning instability. A mode that is localized in regions of unfavor- able magnetic-field curvature and that becomes unstable when the plasma pressure gradient exceeds the local magnetic stress. Banana orbit. An orbit with a banana shape that charged particles can follow in a toroidal magnetic field. Barn. Unit of area used in expressing the cross sections of atoms, nuclei, electrons, and other particles. One barn is equal to 10-24 square centimeter. See also Cross section. Beam-plasma interactions. A beam of energetic particles injected into a plasma usually loses its energy not by colliding with the plasma particles directly but by exciting collective modes of oscillation of the plasma, which then damp into the energy of plasma particles. The general study of such processes is given the name beam-particle interactions. Beta value. Ratio of the outward pressure exerted by the plasma to the inward pressure that the magnetic confining field is capable of exerting. Equivalent to the ratio of particle energy density to magnetic-field energy density. Black holes. The general theory of relativity predicts the existence of a state of gravitational collapse in which the infalling matter ap- proaches infinite density. Because of the time dilation that occurs in general relativity, the collapse appears to an exterior observer to be nearly frozen. No light can escape to the exterior world from the interior of the object thus the name black hole. Black holes of stellar mass, thought to be responsible for accreting galactic x-ray sources, have a radius of about 10 km. Black holes in active galactic nuclei have radii of about 109 km, smaller than the solar system and very much smaller than the surrounding galaxy. Blanket. Region surrounding a fusion reactor core, within which fusion neutrons are slowed down, heat is transferred to a primary coolant, and tritium is bred from lithium. Breakeven. Condition that the fusion power produced in a plasma exceed the power needed to maintain the plasma temperature. For tritium plasmas heated by deuterium beams, the requirements for breakeven are relaxed relative to the Lawson criterion for thermal plasmas. See also Lawson criterion.

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286 GLOSSAR Y Bremsstrahlung. Radiation emitted as a result of deflection (e.g., through near collisions) of rapidly moving charged particles. Brillouin instability. Decay of an intense laser light wave into a scattered light wave and an ion acoustic wave. Occurs in the low-density plasma surrounding an irradiated target in inertial- confinement fusion. Brillouin scattering. acoustic-like plasma wave. Reflection of electromagnetic radiation from an Bumpy torus. A toroidal configuration created from many canted simple-mirror sectors connected together to form a torus. Stability in the regions of unfavorable magnetic curvature is generally provided by hot mirror-confined electron rings. See also Elmo Bumpy Torus. Carbon dioxide laser. A laser that produces intense light pulses in the far-infrared portion of the spectrum, specifically with wavelength of about 10 micrometers. See also Laser. Central cell. The main, cylindrical confinement region of a tandem magnetic mirror. Cerenkov radiation. A distinct type of electromagnetic radiation caused by electrons or protons traveling through or near matter. Charge exchange. Process in which there is a transfer of an electron from a neutral atom to a singly charged positive ion, the latter becoming neutral and the former charged. Charge neutrality. Refers to the strong tendency for a plasma to be everywhere free of net electrical charge. Classical confinement. Best possible limiting case for plasma con- tainment. Only rare collisions between particles are considered as the agent that can lead to cross-field losses. It is also referred to as classical diffusion. Coherent radiation. Monochromatic radiation in which all elements of the wave radiate in unison. Collective accelerator. A device in which charged particles are accelerated in the electric and magnetic fields of neighboring charges. Collective effects. Simultaneous interaction of many charged parti- cles, usually through their Coulomb force. Collision. Close approach of two or more particles, atoms or nuclei, during which such quantities as energy, momentum, and charge may be exchanged. Collisionless plasma. A plasma in which the density is so low, or temperature so high, that close binary collisions have practically no significance, because the time scales of interest are smaller than the collision time.

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GLOSSAR Y 287 Collisionless shock waves. A supersonic airplane creates a shock wave, which causes the familiar sonic boom. The dissipation for ordinary gas-dynamic shocks is due to collisions between gas particles. High-temperature plasmas are collisionless. The dissipa- tion for shock waves in collisionless plasmas is created by micro- scopic collective plasma modes excited in the shock front. Compact toroid. A toroidal confinement configuration utilizing poloidal magnetic fields but no externally produced toroidal field. Its compactness arises from the absence of toroidal field coils linking the doughnut-shaped plasma. Confinement parameter. The product of number density and energy confinement time of a plasma. See also Lawson criterion. Controlled thermonuclear fusion. Process In which very light nuclei, heated to a high temperature in a confined region, undergo fusion reactions under controlled conditions, with the associated release of energy that may be harnessed for useful purposes. Corona. In inertial confinement fusion, refers to the low-density plasma surrounding the irradiated target. In the Sun, it refers to the outermost layer composed of hot, tenuous plasma. See also Solar _ . . . ~ . ~. . corona. Cosmic rays. Cosmic rays are energetic electrons, protons, positrons, neutrons, neutrinos, atomic nuclei, and other particles accelerated by cosmic processes. Some cosmic rays, typically below 109 eV energy, are accelerated at the Sun and in the solar system; those above 109 eV energy are largely accelerated in our galaxy; those above about 1017 eV energy are accelerated outside our galaxy. Cosmic rays, elemental and isotopal abundances. Elemental abun- dance refers to the abundances of different atomic nuclei in the cosmic radiation. Nuclear isotopes refer to atomic nuclei with the same number of protons but differing numbers of neutrons. Studies of these abundances reveal much about the material from which the cosmic rays were accelerated, about how they were accelerated, and the density of galactic material through which they have traveled. Cross section. A measure of the probability that a reaction (nuclear or other) will occur. Usually measured in barns, it is the apparent (or effective) area presented by a target nucleus (or particle) to an oncoming particle. Current drive. The process by which the required toroidal currents are created and maintained in a tokamak. In a conventional tokamak, these currents are maintained for a finite pulse duration by inductive transformer action but then decay. Currents can also be driven

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288 G f OSSAR Y noninductively by radio-frequency waves, in which case they may be maintained indefinitely. Cyclotron frequency. The natural oscillation frequency of a charged particle immersed in an external magnetic field. One mode of oscillation is associated with electron gyrations, the other with ion gyrations. Cyclotron radiation. Radiation emitted at the cyclotron frequency by charged particles in a magnetic field as a result of their natural gyration in that field. Sometimes called synchrotron radiation, especially for very fast particles in which case the-radiation is emitted at high harmonics of the cyclotron frequency. Cyclotron resonance. Resonance absorption of energy from an alter- nating electric field by electrons or ions in a magnetic field when the frequency of the electric field equals the cyclotron frequency. Debye shielding. Departure from the inverse-square law of interaction between point charges caused by the presence of neighboring charges. Deuterium. An isotope of the hydrogen atom with one proton and one neutron in its nucleus and a single orbital electron. Deuteron. The nucleus of a deuterium atom. Diagnostics. Procedure for determining (diagnosis), by one means or another, exactly what is happening inside a plasma during an experiment; also refers to the instruments used for diagnosing. Direct conversion. Generation of electricity by direct recovery of the kinetic energy of the charged fusion reaction products. Direct illumination. Refers to an approach to inertial-confinement fusion in which the target is directly irradiated by laser light. Disruption. A sudden loss of plasma confinement in a tokamak, resulting from inadequate control of kinklike instabilities. Divertor. Component of a toroidal fusion device that provides a magnetic field to divert charged particles in the outer shell of the discharge into a separate chamber where they strike a barrier, become neutralized, and are pumped away. In this way, energetic particles in the outer shell are prevented from striking the walls of the main discharge chamber and releasing secondary particles that would cool the discharge. Double layers. Current-carrying plasmas can create thin layers of strong electric field when the current is sufficiently intense. Since these layers contain sheets of positive and negative charge in close proximity, they have been called double layers. It has been sug- gested that double layers accelerate the particles responsible for the aurora.

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GLOSSAR Y 289 Drift waves. Oscillations in a magnetically confined plasma that arise in the presence of density gradients. Driver. A powerful laser or particle beam used in inertial confinement fusion, to impart energy to a pellet, thereby causing it to implode. Dynamo processes. A term used to describe the process by which the energy in turbulent motions is converted to magnetic-field energy in conducting fluids and plasmas. Earth's magnetic tail. The interaction of the solar wind with the Earth's magnetic field creates a long magnetic tail that extends more than 1000 earth radii downstream of the Earth. The north lobe of the tail contains a magnetic field that is directed toward the Earth, and the south lobe contains flux directed away from the Earth. Jupiter's magnetic tail is at least 5 AU long. EBT. See Elmo Bumpy Torus. ECRH. See Electron cyclotron resonance heating. Electromagnetic radiation. Radiation consisting of electric and mag- netic waves that travel at the speed of light and can be transmitted through a vacuum. Electron. Elementary particle with a unit negative electrical charge and a mass very much smaller than that of the proton. Electron bunching. Formation of tight electron clumps in space and time. Electron cyclotron resonance heating. Mode of heating of a plasma by resonant absorption of energy based on waves induced in the plasma at the cyclotron frequency of the electrons or at harmonics of the cyclotron frequency. Electron density. The number of electrons in a unit volume. Electron-positron pair plasmas. Plasmas with very-high-energy den- sities can contain significant numbers of positrons. Those plasmas that contain only electrons and positrons are called pair plasmas. Pulsars, for example, are thought to generate pair plasmas. Electron temperature. The temperature at which ideal gas molecules would have an average kinetic energy equal to that of electrons in a plasma under consideration. Electron volt (eVJ. Unit of energy equal to the energy acquired by a singly charged particle in passing through a potential difference of 1 volt. 1 eV = 1 .6 x JO- 19 joule. A plasma in which the particles have an average energy of 1 eV has a "temperature" of about 10,000 degrees Celsius. Electrostatic plugging. The use of a positive electrostatic potential in the end cells of a tandem mirror to achieve axial confinement of ions in the central cell.

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290 GLOSSAR Y Electrostatic potential. Refers to the ability of a point or region in a plasma to attract or repel charged particles. For example, a region of positive electrostatic potential will attract electrons and repel posi- tively charged ions. Elm o Bumpy Torus. A magnetic fusion concept in which high-beta electron rings produced by microwave heating stabilize a bumpy torus. End cells. See End plugs. End plugs. Minimum-B mirror cells at the ends of a tandem mirror, within which mirror action provides the dominant mechanism for confining beam-injected energetic ions, thereby providing a positive electrostatic potential for the confinement of thermal ions in the central cell. Energy-confinement time. Time required for a plasma to lose (via radiation or other loss mechanisms) an amount of energy equal to its average kinetic energy. Fertile material. Nuclide that will convert to fissile material on neutron capture and radioactive decay (e.g., uranium-238 or thorium-232. Field-reversed configuration. A confinement configuration of the compact toroid class that utilizes poloidal magnetic fields only. The configuration is generally formed on rapid, dynamic time scales. Fissile material. Any material fissionable by neutrons of all energies, especially including thermal (slow) neutrons as well as fast neutrons (e.g., uranium-235 and plutonium-239. Fluctuations. Refers to small-scale oscillations in a plasma, usually caused by weak instabilities. Flute instability. See Interchange instability. Free-electron laser. A laser that uses free (as opposed to bound) electrons as its active medium. Free-electron radiation source. A source that uses free electrons (from an electron gun, for example) to generate electromagnetic radiation. See also Free-electron laser and Gyrotron. Fusion. Merging of two light atomic nuclei into a heavier nucleus, accompanied in general by the release of energy. Fusion-fission hybrid. Reactor in which energy is produced by both fusion and fission reactions. A fusion neutron source is typically surrounded by a subcritical blanket containing fissile material. If fertile material is also contained in the blanket, the reactor will produce additional fissile material. Gamma rays. Gamma rays are highly energetic photons with energies exceeding about 50,000 electron volts.

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GLOSSAR Y 291 Geometrical optics. A calculational technique for following the prop- agation of electromagnetic waves by tracing the trajectories of rays through a refracting medium, e.g., a plasma. The technique is valid if the wavelength is much shorter than the scale size of the plasma. Guiding center. The average center of rotation of a charged particle in motion in a magnetic field. Gyroirequency. See Cyclotron frequency. Gyroradius. See Radius of gyration. Gyrotron. A device for producing microwave energy that utilizes a strong axial magnetic field in a cavity resonator to produce azimuthal bunching of an electron beam. Also called an electron cyclotron maser. Heavy-ion beams. Beams of ions of heavy elements, such as ura- nium, of gigavolt energies that could be produced in conventional high-energy accelerators and might be used as drivers in inertial- confinement fusion. Helical. Spiraling. Usually refers to the trajectory of a field line or charged particle in a configuration with both toroidal and poloidal magnetic fields. Hertz. Unit of frequency equivalent to one cycle per second. Hohlraur';. A hollow chamber containing electromagnetic radiation in thermodynamic equilibrium with the hot chamber walls. In its application to inertial-confinement fusion, the laser light is shone into the hohlraum and converted into x rays that serve as the pellet driver. Hybrid reactors. See Fusion-fission hybrid. Hydrodynamic efficiency. In inertial-confinement fusion, refers to the fraction of the absorbed driver energy that is delivered in kinetic energy to the fuel. Hydrodynamic instability. A process in inertial-confinement fusion in which nonuniformities in target irradiation may be unstably amplified by hydrodynamic motions of the target surface. Analogous to the instability that develops when a heavier fluid is supported against gravity by a lighter fluid. Hydromagnetic instability. Instability arising from macroscopic mo- tions of a conducting fluid as a result of its interaction with a magnetic field. Hydromagnetics. See Magnetohydrodynamics. ICRH. See Ion cyclotron resonance heating. Ideal MHD. Magnetohydrodynamic model for an assumed infinitely conducting plasma. Ignition. The high-temperature conditions at which the energy depos

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292 GLOSSAR Y ited in a plasma through the fusion process just equals the energy losses. Implosion. A violent inward compression. Impurities. Ions in a fusion plasma of elements other than those of the reacting hydrogenic fuel. Inertial confinement. A dynamic, nonmagnetic plasma confinement scheme that uses compressional inertial forces, e.g., laser radiation compressing a D-T pellet. Interchange instability. The type of hydromagnetic instability in which the plasma interchanges position with the magnetic field; also called a "cute instability," since it would be expected that the interface between the plasma and the magnetic field would become fluted. Inverse bremsstrahlung. A process, inverse to that of bremsstrahlung radiation, in which electromagnetic waves (especially laser light in inertial confinement fusion) are absorbed by collisional defections of electrons vibrating in the wave fields. Also called bremsstrahlung absorption. Ion. Atom that has become charged as a result of gaining or losing one or more orbital electrons. A completely ionized atom is one stripped of all of its electrons. Ion acoustic wave. A longitudinal compression wave affecting the ion density of a plasma. Ion cyclotron resonance heating. Mode of heating a plasma by resonant absorption of energy based on waves induced in the plasma at the cyclotron frequency of the ions or at harmonics of the cyclotron frequency. Ion temperature. The temperature at which ideal gas molecules would have an average kinetic energy equal to that of the ions in a plasma under consideration. Ionization. Process of removing an electron from a neutral atom, thereby creating an ion. Ionosphere (terrestrial, planetaryJ. Ionosphere refers to a layer of plasma in the upper atmosphere that is maintained by photoioniza- tion by solar ultraviolet radiation and by collisions with energetic particles of magnetospheric and solar-system origin. In contrast to magnetospheric plasmas, ionospheric plasmas are collision domi- nated, since they interact with neutral atmospheric gases. Irradiation. Exposure to radiation. Isotope. One of several species of the same element, possessing different numbers of neutrons (but the same number of protons) in their nuclei.

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GLOSSAR Y 293 JET. Joint European Torus, a large tokamak that is owned in common by the European communities. It has been built at the Culham Laboratory and is generally comparable with the TFTR. JT-60. Large Japanese tokamak, currently under construction. It is generally comparable with JET and TFTR but will not use tritium plasmas. Kilometric radiation. Beam-plasma interactions in the auroral accel- eration region excite radio waves with wavelengths of a few kilom- eters thus the term kilometric radiation. Kink instability. Hydromagnetic instability that sometimes develops in a plasma column carrying a strong axial current. The column becomes unstable and undergoes a gross lateral displacement toward the walls of the discharge vessel. Klystron. An evacuated electron-beam tube in which an initial veloc- ity imparted to electrons in the beam results subsequently in density modulation of the beam; used as an amplifier or oscillator for microwave radiation. Krypton-fluoride laser. A laser under development with wavelength of about 0.25 micrometer. See also Laser. Langmair wave. Same as a plasma wave. Larmor radius. See Radius of gyration. Laser. A device that utilizes the natural oscillations of atoms or molecules between energy levels for generating intense, coherent electromagnetic radiation in the ultraviolet, visible, or infrared regions of the spectrum. Laser fusion. Nuclear fusion process that occurs when a small pellet of fuel material is compressed by a burst of laser light. See also Inertial confinement. Laser pumping. A , ~ Use of electron beams or the radiation from intense light sources, for example, to invert the population of lasing mate- rials. Lawson criterion. Condition that the product of number density and confinement time of a plasma (confinement parameter) must equal approximately 10'4 cm3-s at a temperature of about 70,000,000 degrees to produce net power in a fusion reactor. Light-ion beams. Beams of protons, lithium, or carbon ions of a few megavolt energies, generated by pulsed power accelerators and to be used as drivers in inertial-confinement fusion. Light-ion fusion. Inertial fusion concept using light ions (e.g., pro- tons). Limiter. A mechanical structure placed in contact with the edge of a

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296 GLOSSAR Y Microinstabilities. Small-scale plasma instabilities leading to fluctua- tions and anomalous transport. Micrometer. A unit of length used to measure very short distances, such as the wavelength of laser light. One micrometer equals 10-6 meter. Microscopic instability. Short-wavelength, high-frequency instability causing fine-grained plasma perturbations and turbulence. Microwaves. Electromagnetic radiation with a wavelength of a few centimeters or less. Minimum-B configuration. Name given to a magnetic configuration that increases everywhere in strength with increasing distance from the plasma that it is confining. In such a configuration, the plasma finds itself in a region of minimum magnetic potential and is highly stable. Minimum-energy state. Refers to the condition in which a dynamical system, such as a plasma, is in a configuration of minimum potential energy and is therefore highly stable. Mirror. See Magnetic mirror. Mirror ratio. In a magnetic-mirror configuration, the ratio of the strength of the magnetic field at the strongest point on its axis to the weakest field strength between the two magnetic mirrors. Monte Carlo method. A statistical technique for computing the motion of a large number of individual particles. Negative-ion beams. An advanced form of neutral injection in which negative hydrogenic ions (i.e., with excess electrons) are used. The neutralization efficiency, after acceleration to very high energies, is superior to that of positive ions. Neoclassical. Term used to characterize classical-like collisional diffusion in finite toroidal geometries, where geometrical effects enhance the transport relative to an infinitely long straight system. Neodymium-glass lasers. A laser that produces intense light pulses in the near-infrared portion of the spectrum, specifically with a wave- length of about 1 micrometer. See also Laser. Neutral-beam heating. See Neutral injection. Neutral injection. A technique for heating plasmas in which hydrogenic ions are accelerated to high energies, neutralized, in- jected across the magnetic field of a confinement device, and subsequently ionized by the plasma inside the magnetic container. Neutron. Uncharged elementary particle with mass about the same as that of the proton and found in the nucleus of every atom heavier than hydrogen. The energy from fusion reactions appears mainly in the form of energetic neutrons.

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G~OSSAR Y 297 Neutron star. A condensed star that is stabilized against gravitational collapse by the pressure of degenerate nucleons, which are as tightly packed as quantum-mechanical law allows. The centers of large neutron stars are composed primarily of neutrons, with a slight admixture of electrons and protons, whereas the outer layers are an exotic metal composed of neutron-enriched nuclei. Neutron stars are believed to have superstrong magnetic fields. All pulsars are neutron stars, and some galactic x-ray sources are due to mass accretion onto a neutron star from a binary companion star. Nonlinear. Refers to waves and instabilities that have reached am- plitudes at which the disturbances are no longer a small perturbation of the equilibrium. Nonlinear wave. Large electromagnetic perturbation about the plasma equilibrium state. r 1 _~ _ __~ ~ Nonneutral plasma. Plasma composed of a single charged species that is not electrically neutralized by a second species. NOVA. A neodymium-glass laser facility under construction at the Lawrence Livermore National Laboratory. Also, the result of ex- plosion of a star with a mass of about one solar mass. Nucleosynthesis. Nucleosynthesis refers to the processes by which all elements of the periodic table, except hydrogen and helium, are created by nuclear burning in stellar interiors. Numerical models (fluid, MHD, hybrid, kinetic). Numerical models of plasma dynamics can be created at both the fluid and kinetic levels of description. Fluid models solve the equations of fluid dynamics, and MHD models solve the corresponding magnetohydrodynamic equations for magnetized, conducting fluids. Kinetic models follow the motions of the individual electrons and ions in the self- consistently calculated electric and magnetic fields of the plasma. Hybrid models treat the ions kinetically and the electrons as a fluid. Ohmic heating. Heating resulting from the resistance a medium offers to the flow of electric current. In plasmas subjected to ohmic heating, ions are heated almost entirely by transfer of energy from the hotter electrons. Also called Joule heating. Open system. See Magnetic mirror. PBFA II. Particle Beam Fusion Accelerator II, a light-ion beam accelerator to be used in inertial-confinement fusion, under construc- tion at Sandia National Laboratories. Parametric decay. Decay of one wave into two other waves. For example, decay of an intense laser light wave into a plasma wave and an ion acoustic wave. Parametric instability. Instability that occurs in a system whose

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298 G. f OSSAR Y equilibrium is weakly modulated in time or space. The modulation produces a coupling of the linear modes of the unmodulated system and can lead to destabilization. Interaction between three waves, one of which (the pump) feeds energy to the other two. Particle ring. A configuration of the compact toroid class that utilizes energetic ion or electron rings to create a field-reversed configura- tion. Photons. The quanta of electromagnetic radiation. Pinch elect. Constriction of a plasma column carrying a large current, caused by the interaction of that current with its own encircling magnetic field. Pitch angle. The angle that a particle's velocity vector makes with the direction of the magnetic field. See also Loss cone and Pitch- angle scattering. Pitch-angle scattering. A charged particle has a helical orbit in a uniform magnetic field. The pitch of the helix is called the particle's pitch angle, which is determined by the ratio of the components of the particle's velocity parallel and perpendicular to the direction of the magnetic field. When two particles collide, or one particle interacts with a plasma wave, the pitch angles are changed or scattered. Pitch-angle scattering leads to losses of mirror-confined particles in the Earth's radiation belts and in fusion devices. Plasma. Ionized gaseous system, composed of an electrically equiv- alent number of positive ions and free electrons, irrespective of the presence of neutral particles; in view of its prevalence throughout the universe, sometimes called the fourth state of matter. Plasma confinement. Operation intended to prevent, in an effective and sufficiently prolonged manner, the particles of a plasma from striking the walls of the container in which the plasma is produced. Plasma cross section. Refers to the shape of the cross section formed by cutting a doughnut-shaped toroidal plasma. Plasma equilibrium. Plasma system in which there is an overall quasi-steady balance of forces. Plasma frequency. Natural frequency of oscillation of a plasma, caused by the collective motion of the electrons acting under the restoring force of their space-charge attraction to the relatively stationary ions. Plasma instability. State of a plasma in which a small perturbation amplifies, resulting in an alteration of the equilibrium of the system. Plasma radiation. Electromagnetic radiation emitted from a plasma, primarily by free electrons undergoing transitions to other free states

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GO OSSAR Y 299 or to bound states of atoms and ions, but also by bound electrons as they undergo transitions to other bound states. Plasma wave. A disturbance of a plasma involving oscillation of its constituent electrons at the plasma frequency. The term plasma wave is often used more generally to denote collective modes of oscillation in a plasma. Polarization. All electromagnetic radiation can be characterized by its frequency and the direction in which the electric field of the electromagnetic waves oscillates. When the electric field averaged over time takes on all possible directions (relative to the direction of propagation of the radiation), the radiation is said to be unpolarized. When the electric field has a preferred direction, or when its direction rotates coherently, the radiation is polarized. The synchro- tron process, for example, generates polarized radiation. Poloidal diverter. A diverter that takes out poloidal magnetic field lines, forming a separatrix in the poloidal field. See Divertor. Poloidal field. A magnetic field that encircles a toroidal plasma the short way around. See also Toroidal field. Ponderomotive force. Radiation pressure exerted by an electromag- netic wave on charged particles. Positron. The laws of quantum electrodynamics allow the existence of a particle conjugate to the electron, which has the same mass as the electron but the opposite (positive) charge. Proton. Elementary particle with a single positive electrical charge and a mass more than 1800 times larger than that of the electron; the nucleus of an ordinary hydrogen atom. Pulsars. Rapidly spinning magnetized neutron stars that generate beams of electromagnetic radiation, usually radio emissions, in their rotating magnetospheres. These emissions are detected as periodi- cally spaced pulses that repeat at the spin frequency of the parent neutron star. Pulse power system. High-current, high-voltage accelerator that pro- duces short energy bursts by pulse compression. Pumped limiter. An advanced form of mechanical limiter, containing channels through which neutral gas formed by plasma recombination can be pumped away. See also Limiter. Quasars. Quasars appear as unresolved starlike images on astronom- ical photographic plates. However, the spectrum of the radiation that they emit is strongly red shifted, indicating that they are enormously powerful objects located at great distances from our galaxy. Many theoreticians believe that quasars derive their huge luminosity from

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300 GLOSSAR Y accretion onto a massive black hole at the center of an otherwise normal galaxy. REP. See Reversed-field pinch. Radar backscatter. The velocity distributions of the particles that make up a plasma, and the collective fluctuations in the plasma, may be diagnosed by directing a beam of electromagnetic radiation at the plasma and analyzing the backscattered signal. Radar beams backscattered from the ionosphere have provided valuable informa- tion about ionospheric plasmas. Radiation. Emission and propagation of energy by means of electro- magnetic disturbances that display wavelike behavior. See also Plasma radiation. Radio-frequency heating. A technique for heating plasmas by the absorption of the energy contained in electromagnetic waves launched by an antenna or waveguide into the containment vessel. Various types of radio-frequency oscillators can provide the power sources for such techniques. Radio galaxies. That class of active galaxies that radiate significant quantities of energy in the form of radio emissions. A typical radio galaxy has twin emission lobes aligned along a line that passes through the center of the parent galaxy. Radius of gyration. For a charged particle moving transversely in a uniform magnetic field, the radius of curvature of the projection of its path on a plane perpendicular to the field. Also known as the Larmor radius. Raman instability. Decay of an intense laser light wave into a scattered light wave and a plasma wave. Occurs in the low-density plasma surrounding an irradiated target in inertial-confinement fu- s~on. Ray tracing. See Geometrical optics. Rayleigh-Taylor instability. See Hydrodynamic instability. Reconnection. That class of plasma processes by which magnetic- field topologies which would be stable in MHD change owing to collisional or collective dissipation. The simplest case is a plane neutral sheet in which the magnetic field lines are oppositely directed above and below the neutral layer, where the magnetic field is zero. Reconnection causes the originally oppositely directed field lines to pass through the neutral sheet and connect to one another. Recon- nection converts magnetic energy into particle energy. Refraction. Bending of oblique incident rays as they pass from a medium having one refractive index into a medium having a different refractive index.

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GLOSSAR Y 301 Relativistic particles. Classical Newtonian mechanics becomes in- valid for particle velocities approaching the speed of light, where Einstein's theory of relativity must be used. Particles whose veloc- ities are close to the speed of light are said to be relativistic. Resistive instability. Instability resulting from macroscopic motion of a plasma with a finite electric conductivity. Resistive instabilities are generally much weaker than hydromagnetic instabilities. Resistive MHD. Magnetohydrodynamic model that allows for finite plasma resistivity. Reversed-field pinch. A toroidal configuration that utilizes the pinch effect to confine a plasma carrying a large toroidal current and provides stability against kink and interchange modes by introducing a relatively weak, externally imposed toroidal magnetic field that reverses its direction at the edge of the plasma. Rotational transform. A toroidal magnetic-field configuration is said to possess rotational transform if the lines of force, after one circuit around the configuration, do not close exactly on themselves but are rotated through some angle called the rotational transform angle. Safety factor. The inverse of the rotational transform of a toroidal magnetic-confinement system. In a tokamak, the value of the safety factor must exceed unity to avoid kink instabilities. Scattering. Deflection of one particle as a result of collisions with another. Second stability regime. Refers to a regime of toroidally confined plasmas that is free from ballooning instabilities at arbitrarily large beta values. Separatrix. A magnetic surface that separates regions of closed field lines from regions of open field lines. Solar corona. The solar corona is the tenuous, outermost plasma atmosphere of the Sun. It is heated to a temperature of about 1 million kelvins and extends to a distance of several solar radii from the optical surface (photosphere) of the Sun. It ultimately blends into the solar wind. Solar coronal holes. Solar coronal holes are open magnetic-field configurations in which the corona in the solar wind is generated. Since the plasma inside the open configuration is cooler than in the surrounding corona, it emits less x radiation. Thus, the open regions appear as holes in generally bright x-ray photographs of the Sun. Solar coronal loops. Closed magnetic structures in the solar corona, which take the characteristic form of loops emerging from the visible surface of the Sun. Solar pare. A solar flare is believed to be triggered by the rapid

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302 GLOSSAR Y conversion of magnetic to particle energy in the solar corona. Strong flares generate intense radio, optical, and x-ray emissions and launch strong shock waves in the solar wind that propagate throughout the solar system. Solar photosphere. The outermost layer of the Sun, as observed in optical light. It is the region from which most of the Sun's optical photons escape into space. Solar (stellarJ chromosphere. A thin layer between the photosphere and corona, in which atomic spectral lines can be observed in emission rather than absorption. Solar-terrestrial physics. The study of the chain of processes, almost all of which involve plasma physics, that links the generation of the Sun's magnetic field deep inside the Sun, solar surface and coronal magnetic activity, generation of the solar wind, the dynamics of the Earth's magnetosphere, and the Earth's ionosphere and upper atmosphere. Solar wind. A supersonic, super-Alfv~nic plasma wind that is gener- ated in open magnetic structures in the solar corona (solar coronal holes) and streams throughout interplanetary space. Soliton. Large-amplitude wave pulse that preserves its amplitude and shape. Space charge. interact. Spheromak. A confinement configuration of the compact toroid class that utilizes toroidal and poloidal magnetic fields of comparable magnitude but differs from the tokamak and reversed-field pinch in that the toroidal field is produced entirely by currents within the plasma. The configuration is generally formed by quasi-static induc- tive techniques. Stellarator. A toroidal confinement configuration that uses the com- bination of a toroidal magnetic field and an additional field created by helical windings. This magnetic configuration provides a rotational transform in itself and permits containment in the absence of an axial current in the plasma. Strongly coupled plasma. Dense plasma in which the electrical energy is comparable in magnitude with the particle kinetic energy. Substorms. Rapid how reconfigurations of the Earth's magnet- osphere, which are thought to be induced by changes in the direction of the solar-wind magnetic field and triggered by reconnection in the magnetic tail. Substorms greatly intensify the aurora in the upper atmosphere. Sunspot. Cool dark regions observed optically in the solar Local electric fields through which charged particles

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GLOSSAR Y 303 photosphere. Sunspots contain concentrations of strong magnetic fields that connect to the corona above the photosphere and to the solar convection zone below. Super-Alfvenic. Alfven wave. A plasma flow speed that exceeds the speed of an Superconductor. Type of conductor that permits an electrical current to flow with zero resistance. Superconducting coils are expected to be used as electromagnets in most types of fusion reactors. Supernova. An explosion of an entire star. Supernova remnant. The remains of a star's outer layers that are observed to expand at high speeds outward into space following a supernova explosion. Superthermal electrons. Electrons in a plasma that have been accel- erated by dc or fluctuating electric fields to energies much greater than the average thermal energy, or temperature, of the plasma. Synchrotron radiation. Relativistic particles in a magnetic field emit a broadband continuous spectrum of synchrotron radiation, which was first observed in high-energy particle accelerators, called synchro- trons. Tandem mirror. A magnetic-mirror configuration in which two minimum-B mirror cells are used to plug the ends of a much larger, cylindrical central mirror cell. Target. In inertial-confinement fusion, refers to the pellet of D-T fuel that is to be imploded by the laser light or particle beam. Tearing instability. Resistive instability that grows at a rate slower than the MHD rate but faster than the skin diffusion rate. The instability tears poloidal field lines and reconnects them into a new state of lower magnetic energy. Terrella. Terrella means little Earth. The term is used to refer to laboratory experiments designed to simulate the interaction of the solar wind with the Earth's magnetosphere, in which a high-speed Rowing plasma is directed toward a dipole magnetic field. TFTR. Tokamak Fusion Test Reactor, a large toroidal device at Princeton Plasma Physics Laboratory operating as a standard tokamak capable of modest adiabatic compression. In addition to operating as a hydrogen experiment, it will be capable of injecting high-energy neutral deuterium into a tritium plasma, thereby produc- ing a D-T plasma under breakeven reactor conditions. Thermal barrier. Regions, located between the central cell and the end plugs of a tandem mirror, in which the electrostatic potential is driven negative by local electron heating and which serve to isolate the electrons in the central cell from those in the end plugs, thereby

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304 GLOSSAR Y increasing the efficiency with which the positive electrostatic plug- ging potentials can be maintained. Thermal conductivity (di;ffusivity>. Quantities that measure the rate at which energy (heat) can be transported across (or along) the mag- netic field in a plasma with a temperature gradient. Thermonuclear burn. The process in which alpha particles from D-T fusion reactions can sustain the plasma temperature, thereby pro- longing the reacting conditions until much of the D-T fuel is consumed. Thermonuclear conditions. Achievement of an adequately confined plasma having temperature and density sufficiently high to yield significant release of energy from fusion reactions. Tokamak. Name given to a specific magnetic-field geometry in con- trolled fusion, involving confinement and heating of a plasma in a toroidal configuration. A large current induced in the plasma pro- vides the rotational transform necessary for confinement while simultaneously heating the plasma. Toroidal confinement. Name given to the general class of doughnut- shaped magnetic fields in which lines of force close on themselves. Stellarators, tokamaks, and reversed-field pinches are examples of this class of devices. Toroidal field. The main confining magnetic field, which encircles a toroidal plasma the long way around. See also Poloidal field. Toroidal-field coils. Coils in a toroidal system that provide the main confining field. Each turn completely surrounds the minor axis of the plasma. Torsatron. A modification of the stellarator concept, in which both toroidal and poloidal fields are generated by helical windings alone. Transmission line. High-voltage coaxial system used to transmit high-voltage pulsed currents from the accelerator to the plasma target. Trapped-particle instability. Slowly growing class of instabilities driven by particles that cannot circulate freely in a toroidal system. Trapped particles. Those particles in a toroidal configuration that are unable to circulate freely around the torus but are reflected from regions of relatively high field, as in a magnetic mirror. The term also applies to particles trapped in the Earth's dipole field. Tritium. An isotope of the hydrogen atom with one proton and two neutrons in its nucleus and a single orbital electron. Triton. The nucleus of a tritium atom. Turbulence. Random mixing of large electromagnetic perturbations.

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GLOSSAR Y 305 Velocity space instability. A class of instabilities driven by particle distributions that are not in thermal equilibrium Waveguide. A device for transmitting relatively short-wavelength electromagnetic waves into a plasma-containment vessel. Wavelength. The length of a wave measured from one point to the corresponding point on the next wave, usually measured from crest to crest. Wave-particle interactions. The interactions between particles and collective modes of oscillation (waves) in a plasma are wave-particle interactions. These lead to much more dissipation than do collisions, when the plasma temperature exceeds about 10,000 kelvins. X ray. A form of electromagnetic radiation emitted either when the inner orbital electrons of an excited atom return to their normal state or when a metal target is bombarded with high-speed electrons. Z-pinch. Plasma that pinches because of high electrical currents and self-magnetic fields.

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